The present invention relates to a ceramic substrate with a metal plate and a method of producing the same. More particularly, the invention is concerned with a ceramic substrate with a metal plate suitable for use as the substrate for a hybrid semiconductor integrated circuit, as well as to a method of producing such a substrate.
Generally, a semiconductor device referred to as "hybrid integrated circuit" has an insulation plate of a ceramic material, a metal plate constituting wiring laid on one side of the insulation plate, and semiconductor elements and other circuit elements placed on the metal plate. In order to efficiently remove the heat generated in the circuit elements, the semiconductor device often employs heat radiating plates or fins attached to the other side of the insulation plate.
The production of such a hybrid integrated circuit encounters various technical subjects or difficulties to be overcome.
A first technical subject is to eliminate breakdown of the semiconductor elements attributable to the fact that the circuit elements, particularly the semiconductor elements, metal plate and the insulation plate have different values of thermal expansion coefficient.
A second technical subject is to achieve a high reliability of the semiconductor device by making sure of the bonding between parts of different materials.
A third technical subject is to efficiently dissipate the heat generated in the circuit elements thereby to avoid malfunction of the circuit elements due to overheat.
Hitherto, as a measure for achieving the first technical subject mentioned above, it has been proposed and actually conducted to dispose, between the semiconductor elements and the metal plate, a buffer plate made of a metal having a thermal expansion coefficient approximating that of the semiconductor element, e.g. molybdenum, tungsten, Fe-Ni alloy and so forth. This way of solution, however, undesirably imposes a difficulty in achieving the third technical subject, i.e. the efficient dissipation of the heat, partly because the buffer plate and the bonding layers increase the length of path of heat to be dissipated and partly because the buffer plate itself has a large thermal resistance. In addition, the semiconductor becomes less reliable and the yield is decreased due to increase in the number of parts employed and the number of steps of the production process.
As a measure for simultaneously achieving the first to third technical subjects to some extent, it has been proposed to interpose, between the semiconductor elements and the insulation plate, a composite material produced by embedding carbon fibers in a copper matrix. This copper-carbon fibers composite material has simultaneously both of high thermal/electric conductivities inherent to the copper and small thermal expansion peculiar to carbon fibers advantageously. In addition, the thermal expansion coefficient can be varied as desired by varying the carbon fibers content. Therefore, the use of copper-carbon fibers composite material interposed between the semiconductor elements and the insulation plate offers the following advantages over the known construction which employs the buffer plate mentioned above.
(1) The problem attributable to the difference in thermal expansion coefficients between the semiconductor elements and insulation plate can be overcome solely by the use of the composite material. PA1 (2) As a result, the thickness of the insulation plate can be decreased. PA1 (3) The thermal resistance of the device as a whole can be decreased because it is possible to eliminate the buffer plate which has large thermal resistance and to thin the insulation plate which also has large thermal resistance. PA1 (4) The reliability of bonding can be improved because the number of parts and, hence, the number of portions requiring bonding are decreased.
The bonding between a metal plate and an insulation plate such as of a ceramics material is usually conducted by a method in which films of a metal wettable to the bond are formed on both bonding surfaces by evaporation, plating, metallizing or the like technic. For instance, when solder consisting mainly of lead is used as the bond, films of Ni, Cr-Ni-Ag alloy or the like material are formed beforehand on the bonding surfaces of the metal plate and the insulation plate, and thereafter both are bonded by the solder. Thus, since the bonding is made through at least three metallic layers all of which are generally made of materials having large thermal resistance, it is impossible to sufficiently reduce the thermal resistance between the semiconductor elements and the insulation plate.
Thus, in order to reduce the heat dissipation resistance so as to increase the capacity of a hybrid integrated circuit, it is most important among others to reduce the thermal resistance in the bonding layer between the metal plate and the ceramic plate.